Conventionally, in color image forming apparatuses, such as a color laser printer, a method of transferring a toner image formed on a surface of a photosensitive drum to a sheet transported by a sheet transport belt, and a method of once transferring a toner image formed on a surface of a photosensitive drum to an intermediate transfer belt and then transferring it to a sheet therefrom have been known.
A belt, such as the sheet transport belt and the intermediate transfer belt, is wound around a plurality of rollers, and arranged so as to come in contact with the surface of the photosensitive drum. Therefore, the contact of the belt with the photosensitive drum causes a toner to be shifted and adhered to the surface of the belt, and the contact of the belt with a sheet causes a sheet dust to be adhered thereon.
A color image forming apparatus including such a belt is provided with a cleaning unit for removing the adherent on the surface of the belt. As the cleaning unit, one that makes a cleaning blade or a fur brush contact with the surface of the belt and then physically removes the adherent thereon has been known.
Another structure of the cleaning unit can be considered in which a cleaning roller is arranged in contact with the surface of the belt and a bias is applied to the cleaning roller, so that a potential difference is produced between the cleaning roller and the belt, whereby an electrostatic force causes the adherent on the surface of the belt to be shifted to the cleaning roller.
However, under the effect of the bias applied to the cleaning roller, the belt is charged, and due to the charging (uneven charging), a white patch (phenomenon in which an image is not locally printed) may appear in an image formed on a sheet.